As Vice President Joe Biden’s task force introduces ideas into the gun control debate, The Post’s Phil Rucker describes what executive orders President Obama can pass and what must wait for Congress. (The Fold/The Washington Post)

President Obama on Wednesday will formally announce the most aggressive and expansive national gun-control agenda in generations as he presses Congress to mandate background checks for all firearms buyers and prohibit assault weapons and high-capacity ammunition clips.

The announcement will set off a fierce confrontation with Congress over an issue that has riven American society for decades. Obama’s far-reaching firearms agenda has at best tepid support from his party leaders and puts him at loggerheads with Democratic centrists.

Days before his second inauguration, Obama is seeking to drive the gun debate in a way that contrasts with the accommodating approach he often took during his first term. In the weeks ahead, he will attempt to rally popular support to bend the will of lawmakers to vote for what he considers the ideal, not merely the possible.

“Yes, we can reduce gun violence, but it’s something we have to do together,” White House press secretary Jay Carney told reporters Tuesday. “It’s something that cannot be done by a president alone. It can’t be done by a single community alone or a mayor or a governor or by Congress alone. We all have to work together.”

Obama will begin this effort Wednesday in the presence of children who wrote him letters after last month’s mass shooting at a grade school in Newtown, Conn., and who have been invited to Washington to attend the rollout.

In addition to background checks and restrictions on military-style guns and ammunition magazines, Obama is expected to propose mental health and school safety initiatives such as more federal funding for police officers in schools, according to lawmakers and interest group leaders whom White House officials briefed on the plans.

Bruce Reed, Vice President Biden’s chief of staff, told liberal activists late Tuesday that Obama’s package would also include a federal gun trafficking measure to stop straw-man purchases and crack down on trafficking rings after a number of mayors raised the issue, said a person familiar with the plan.

Obama also is expected to present up to 19 executive actions that his administration will take, the lawmakers and advocates said. These steps include enhanced federal scientific research on gun violence and a modernized federal database system to track guns, criminals and the mentally ill.

Most of these actions are relatively narrow in scope, however, and experts have said that without accompanying legislation they will do little to curb gun violence, at least in the near term.

Asked about the constraints on Obama’s executive powers, Carney said, “It is a simple fact that there are limits on what can be done within existing law.”

After Biden led a month-long task force, Obama decided to push an expansive agenda that in many ways represents his liberal base’s wish list rather than proposals that may be more politically viable to a divided Congress.

Obama’s proposals amount to the most comprehensive federal regulations of the firearms industry since 1968, when President Lyndon B. Johnson acted in the aftermath of high-profile assassinations.

“My starting point is not to worry about the politics,” Obama said. “My starting point is to focus on what makes sense, what works, what should we be doing to make sure that our children are safe and that we’re reducing the incidents of gun violence.”

Lawmakers, he added, “are going to have to have a debate and examine their own conscience.”

Already, there are warning signs about the hurdles Obama’s agenda may face on Capitol Hill. Senate Majority Leader Harry M. Reid (D-Nev.) said it would be exceedingly difficult to pass an assault-weapons ban, which appears to be the most polarizing of Obama’s proposals.

“Let’s be realistic,” Reid told a Nevada PBS affiliate last week. “In the Senate, we’re going to do what we think can get through the House, and I’m not going to go through a bunch of these gyrations just to say we’ve done something.”

House Democratic Whip Steny H. Hoyer (Md.) echoed that calculation on Tuesday by acknowledging the difficulties that gun-control legislation would face in the Republican-led House.

“That’s been the case based on past history,” Hoyer told reporters.

More than half of all Americans say the Newtown shootings have made them more supportive of gun control, according to a Washington Post-ABC News poll released Monday. An assault-weapons ban has the support of 58 percent of Americans, the poll shows.

Congress will take up the federal proposals next week — first in the Democratic-controlled Senate and then the House.

Gun control will be only one point of friction between the White House and the Capitol. Policy fights loom over raising the nation’s debt ceiling as well as overhauling immigration laws.

Obama’s gun control proposals are sure to face stiff opposition from the National Rifle Association, which released a video Tuesday on its Web site calling Obama an “elitist hypocrite” for having the Secret Service protect his daughters at school while voicing skepticism about an NRA effort to place armed guards in all schools.

Even some of the administration’s allies on Capitol Hill, including some rural Democrats, have criticized parts of Obama’s agenda.

“An assault-weapons stand-alone ban on just guns alone, in the political reality we have, will not go anywhere,” Sen. Joe Manchin (D-W.Va.) said Sunday on CNN.

Lawmakers who have been part of Biden’s discussions said the White House is well aware of the political difficulty it faces in advancing this agenda.

“I think there’s a commitment to do the big things,” said Rep. Jackie Speier (D-Calif.). “I also think that they’re realists, and in addition to doing the big things, they want to make sure that they do as many of the effective things that we can find some level of consensus on.”

Consensus appears more possible around universal background checks and a ban on magazines capable of carrying more than 10 rounds of ammunition.

Rep. Carolyn McCarthy (D-N.Y.) said she has spoken discreetly with several Republican lawmakers who may be open to backing a ban on high-capacity clips. “What I said to them is, ‘Do your own press conference. Come out as a group. There’s power in numbers,’ ” she said.

Some gun-control advocates say universal background checks could do more to stem gun violence than an assault-weapons ban because they would keep more firearms — including handguns used in most shootings — out of the hands of criminals or those with mental illnesses.

Matt Bennett, a senior vice president at Third Way, a centrist think tank consulted by Biden’s task force, said of the assault-weapons ban: “We support it, but we don’t think it will be easy to do. And we’re not sure that it is worth the expenditure of a tremendous amount of political capital to get.”

The long-dormant debate over gun laws was revived in December after the shootings at Sandy Hook Elementary in Newtown.

Obama and his allies plan to pressure Congress from the outside, just as they did in the recent “fiscal cliff” negotiations that resulted in tax increases for the wealthiest Americans.

“The president can play a vital role in rallying the public support that already exists,” said Dan Gross, president of the Brady Campaign to Prevent Gun Violence. “It’s not a question of muscling anything through. It’s a question of changing the political calculus on this issue.”

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